The analysis and quantification of blood components is an important diagnostic tool for physicians. Although good noninvasive blood analysis technology is not currently available, blood samples still need to be obtained from a great number of patients every day. A well known example is home monitoring of glucose levels by a diabetic individual. Typically, such a person would prick a finger to obtain a drop of blood and then manually transfer it to an analysis strip. Unfortunately, currently available auto-pricking devices induce significant pain, which in turn results in low patient compliance for self testing. Much effort has been devoted to developing devices that would result in less pain in finger pricking. Pain reduction in blood sampling is believed to lead to increased patient compliance of monitoring and treatment regimens, thus improving disease management, resulting in lower long-term treatment costs.
A trend in modern blood collection methods has been to collect smaller sample volumes. Conventional blood sampling methods require a drop of blood to form on the surface of the skin, which is difficult to achieve with small incisions yielding small sample volumes. Miniaturization of the pricking element in an effort to reduce pain upon lancing, and hence generating small sample volumes, requires a compatible blood collection and storage apparatus.
(a) Mechanical Phenomenon of Skin Rupture
To successfully obtain blood, a piercing device must traverse the skin's various layers to reach the blood vasculature. Human skin is composed of a tough, keratinized squamous epithelium. The outermost layer of skin is known as the epidermis (100 microns thick), and has its own distinct layers: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. (For a review about skin, see Tortua and Anagnostakos "Principles of anatomy and Physiology," Harper and Row 1981). The innermost layer consists of the dermis, which is 2-5 microns thick. Because of its varying elasticity and the thickness due to the cellular structure and anatomical locations, the force necessary for penetrating the epidermis to access the vascular beds within the dermis layer will vary. It has been reported that skin tension is the greatest in the areas where the epidermal elastic keratinous fibers are dense, particularly in regions where the skin is thick, such as is found in the epigastric (stomach) regions.
The amount of force necessary to penetrate the skin surface will depend on the force applied normal to the surface of the skin needed to exceed the rupture strength. There exists an elastic range within which the degree of deflection corresponds directly with the applied force (skin depression). When the rupture limit is exceeded, a non-linear response by the skin (otherwise known as the inelastic response) occurs, corresponding to the further stretching of the skin at the point of application prior to rupture. The applied force reaches a maximum when the skin ruptures, resulting in the penetration of the object into the skin (see F. R. Shanley "Strength of Materials," McGraw Hill, 1957). The capillary bed under the dermis is approximately 300 to 750 microns below the outer surface of the skin in the areas of the fingers, the forearms and the stomach. Bleeding occurs when the penetration of the object reaches the capillary bed.
(b) Minimization of Pain
Pain in blood sampling due to the skin being pierced is thought to be generated through pressure waves that are built up at the site of puncture. Minimizing the incision angle of penetration, and hence pressure wave buildup, would greatly reduce the pain sensation on sampling, thereby reducing patient anxiety and the reluctance to self test.
In normal patients, an acceptable threshold for mechanically caused pain in the intact skin is 0.7 g +/-0.06 g over an area of 491 square microns, for a piercing object with a tip diameter of 25 microns (see Dash and Deshpande "Pain and itch sensations: specific chemosensory mechanisms in the human skin," The Somatosensory System, Theme edition/PSG 1975). A successful method to minimize pressure waves generated through skin puncture by a needle, pin or lancet, would be to minimize the area over which the puncture occurs. This can be achieved by miniaturizing the needle or lancet, provided the force applied to create the wound is small. The smaller the needle, the less force is required to puncture the skin, and less nerves endings are stimulated by the cut. In addition to the force required to penetrate the skin, the depth of penetration affects the sensation of pain as well.
Current methods for blood sampling (e.g. self testing at home) involve the cutting or slicing action of a lancet type device. The state-of-the-art sampling devices use spring-driven lancets to puncture the skin. Examples of such devices are described in Meinke (U.S. Pat. No. 4,442,836), Burns (U.S. Pat. No. 4,535,769), Morita (U.S. Pat. No. 5,314,442), and Jorgensen (U.S. Pat. No. 5,439,473). Also, O'Brien (U.S. Pat No. 4,924,879) describes a blood lancet device that results in reduced pain in blood sampling while obtaining a sufficient amount of blood through shaping of the wound. This device indexes the lancet position such that different portions of the skin are punctured on each use. It is noticeable, however, that such devices do not address the problem of significantly reducing the pain on incision as a function of wound size, as well as pressure applied. Burns (U.S. Pat. No. 5,395,387) describes a lancet assembly incorporating a cutting edge that is designed to reduce pain through an increase in the shear percentage of the blade (increase in the ratio of the length of the cutting edge to the total inserted blade length). However, such a device can still lead to significant amount of pain if an incision is made large enough to collect a large drop of blood.
One disadvantage of miniaturization of conventional lancets is fragility, which might lead to breakage of the lancet in the wound. Pain (which people universally want to avoid) is one reason for non-compliance of home blood sampling. Thus, there is a need for a blood sampling device that can be used with minimal or no pain, and yet is capable of providing and transferring an adequate amount of blood either to a test strip or to a storage area.